I want to use cat filepath > /dev/null as a cheap memory caching mechanism.
What I am wondering about is: if I call it a second time, and the file
is already in the disk cache, is the OS smart enough to do nothing?
Update: I've tested this on a CIFS volume, using fadvise POSIX_FADV_WILLNEED to cache the file locally (using linux-ftools on command line). Turns out that the volume needs to be mounted in read-write mode for this to work. In read only mode, the fadvise seems to be ignored. This must have something to do with the samba oplock mechanism.
It is better to posix_fadvise(...,POSIX_FADV_WILLNEED) than to cat the file to /dev/null - it requires less actual IO, and doesn't need the file contents to be read into userspace ram, destroying CPU caches.
Moreover, if the relevant part of the file is already in the cache, the posix_fadvise will proabably do a lot less work than cat file > /dev/null
If you feel that you really need the pages to be in core just now, then mmap the relevant section of the file and mlock it (unlock it afterwards; it might get discarded immediately if memory pressure is tight). That needs root privileges.
In general doing this kind of thing is a pessimisation and should be avoided, however. Forcing the kernel to behave how you want may reduce its ability to optimise the actual workload just now.